The Clinical Core, will serve as a reservoir of well-characterized study subjects, both patients and controls, for behavioral, clinical and basic research studies. As such, the Clinical Core will serve as the source of patients for Chronic Fatigue Syndrome Clinical Research Center (CFS CRC). This Core has 4 major specific aims 1) to prospectively evaluate and follow a referral clinic-based cohort of patients with chronic fatigue and CFS; 2) to utilize this population as the basis for investigations on CFS; 3) to maintain databases and banks of biological specimens on a variety fatigued and control populations and to recruit new comparison groups to improve our understanding of CFS; and 4) to examine the operating characteristics of clinical evaluation instruments already collected and patient subgroups using available data. The results of a comprehensive evaluation, including information on demographic, medical, psychological, functional and social features on almost 1,200 patients seen in a referral clinic are currently in our data base. Ethnic/racial minorities comprise about 8% and women 77% of patients. Information on new patients is entered weekly and patients are re- evaluated periodically. Control groups available for comparison to CFS patients include healthy individuals and those with medical disorders post-mononucleosis fatigue, the symptom of fatigue but not CFS, major depression, multiple chemical sensitivities, rheumatoid arthritis, fibromyalgia and temporomanidubular joint disorder. Besides descriptive studies, other questions amenable to study using the Clinical Core include the development of a battery of appropriate assessment measures for use in CFS and the evaluation of diagnostic tests or objective markers. In fact, the use of the Clinical Core resources has resulted in the development of a promising test for CFS and in the submission of 5 R01 and many other grant applications.